This invention relates to a process for the preparation of a powder coating composition containing a copolymer of a glycidyl group- or methylglycidyl group-containing unsaturated monomeric compound with an ethylenically unsaturated comonomer.
Powder coating compositions are now used in a wide variety of fields in place of liquid coating compositions which cause, during coating and hardening, air pollution by the generation of organic vapors. In particular, powder coating compositions containing a glycidyl group-containing acrylic resin and a polybasic acid curing agent are advantageously used because they give coatings having a beautiful appearance and excellent weatherability.
One known method for the preparation of such a glycidyl group-containing acrylic resin powder coating composition includes suspension-, bulk- or solution-polymerizing a glycidyl group-containing unsaturated monomeric compound with an ethylenically unsaturated monomer to obtain an acrylic copolymer-containing product. The product is treated for the removal of volatile matters such as a solvent and is subsequently coarsely pulverized and then dry-mixed with a curing agent, a pigment and other additives using a mill. The milled mixture is then melted, kneaded, pelletized, ground and sieved to obtain the powder coating composition.
The known method, however, has the following drawbacks. Namely, when suspension polymerization is adopted in the above method, a large amount of a chain transfer agent must be used in order to obtain a relatively low molecular weight copolymer. When a bulk polymerization is adopted, on the other hand, there is caused a drawback that a high polymerization degree cannot be attained. With a solution polymerization using an inert solvent such as toluene or xylene, a step for removing the solvent from the polymerization product must be performed at a high temperature under a high vacuum for a long period of time.
Additionally, since the above acrylic copolymer obtained by the removal of volatile matters is in the form of a bulk at a low temperature, it is necessary to coarsely pulverize same before mixing with the curing agent, pigment and additives. Further, kneading, pelletizing and grinding of the mixture must be carried out to obtain a powder coating composition.
It is, therefore, an object of the present invention to provide a process for the preparation of a powder coating composition containing a glycidyl group- or methylglycidyl group-containing acrylic resin, which is devoid of the drawbacks of the above conventional process.
Another object of the present invention is to provide an economical method for the preparation of a powder coating composition of the above-mentioned type.
In accomplishing the foregoing object, the present invention provides a process for the preparation of a powder coating composition, comprising copolymerizing 5-90% by weight of at least one monomeric compound selected from the group consisting of glycidyl group-containing unsaturated compounds and methylglycidyl group-containing unsaturated compounds with 95-10% by weight of an ethylenically unsaturated monomer in methanol to obtain a resin solution; and
spray-drying said resin solution to obtain a powdery resin.
In another aspect, the present invention provides a process for the preparation of a powder coating composition comprising the steps of:
copolymerizing 5-90% by weight of at least one compound selected from the group consisting of glycidyl group-containing unsaturated compounds and methylglycidyl group-containing unsaturated compounds with 95-10% by weight of an ethylenically unsaturated monomer in methanol to obtain a resin solution;
feeding said resin solution to an extruder having a die and a plurality of vent ports;
while removing volatile matters existing in said resin solution in said extruder from said extruder through said vent ports, extruding said resin solution through said die to obtain extrudates; and
pulverizing said extrudates.
Other objects, features and advantages of the present invention will become apparent from the detailed description of the preferred embodiments of the invention to follow.